The production of polymers and copolymers of lower .alpha.-olefins, particularly ethylene and propylene, has gained substantial commercial importance. The polymeric products are relatively inexpensive and exhibit a number of commercially useful properties. In the case of the polymerization of ethylene, the process is relatively uncomplicated in that the product type is not influenced by the manner in which the ethylene molecules add to the growing polymeric chain during polymerization and the polymeric product does not exist in stereoisomeric forms.
In the case of polypropylene, however, the presence of pendant methyl groups on the polymeric chain provides the possibility of several product types, depending on the steric regularity with which propylene molecules add to the growing chain. Much if not most of the commercial polypropylene results from the stereoregular addition of propylene molecules in a regular head-to-tail manner. The form of polymer having a substantial proportion of random addition of propylene units is termed atactic and this amorphous form is less desirable. If present in a significant proportion, the atactic polymer must be removed as through an extraction to provide the more desirable crystalline material. Also significant from a commercial standpoint is the activity of the polymerization catalyst. A number of the early polymerization catalysts, e.g., trivalent titanium, chromium or vanadium catalysts, were of a relatively low activity and the polymeric product contained significant proportions of catalyst residues. The removal of such residues as by a deashing step was required in order to obtain commercially satisfactory properties.
The more recent olefin polymerization catalysts, generally based on titanium, are more stereoregulating and of sufficient activity so that extraction and deashing steps are not required. In terms now employed conventionally, the high activity catalysts are formed from a solid procatalyst which typically contains magnesium, titanium and halide moieties, a cocatalyst which is usually an organoaluminum compound and a selectivity control agent which may be provided as a partial or total complex with the cocatalyst. Although each of these three components has a significant influence on the polymerization catalyst and process, as well as the polymer thereby produced, the nature of the catalyst and the polymer product appears to be most influenced by the procatalyst. Much of the research directed toward the improvement of olefin polymerization catalysts has been devoted to improvement of the procatalyst.
Kioka, et al, U.S. Pat. No. 4,330,649, describe a solid catalyst component (procatalyst) obtained by heating a soluble magnesium compound such as magnesium chloride with a higher alcohol in the presence of an ester to produce a solution. This solution is added to titanium tetrachloride and an electron donor to form the procatalyst. Band, U.S. Pat. No. 4,472,521, reacts a magnesium alkoxide, wherein each alkoxide has four or more carbons, in the presence of an aromatic hydrocarbon. Titanium tetrachloride and an electron donor are added to the resulting solution to form a solid procatalyst which is post-treated with transition metal halide. Arzoumanidis, U.S. Pat. No. 4,540,679, produces an olefin polymerization catalyst component by contacting a suspension of magnesium ethoxide in ethanol with carbon dioxide. The addition of organoaluminum in hydrocarbon results in the formation of granular particles which are employed as a support for a titanium compound upon contact with titanium tetrachloride. Nestlerode et al, U.S. Pat. No. 4,728,705, solubilize magnesium ethoxide in ethanol with carbon dioxide and spray dry the resulting solution or use the solution to impregnate carrier particles. The solid particles resulting from either modification are useful in the production of a procatalyst of desirable morphology.
A somewhat different process is described by Job, U.S. Pat. No. 4,710,428, wherein a magnesium compound of the general formula EQU Mg.sub.4 (OR).sub.6 (ROH).sub.10 A (I)
is formed wherein R independently is lower alkyl and A is at least one anion having a total oxidation state of -2. This magnesium complex is reacted with a tetravalent titanium halide, a halohydrocarbon and an electron donor to form the procatalyst. The use of such magnesium compounds has certain advantages in that they are crystals of desirable morphology in contrast with magnesium ethoxide which is not. The crystals are converted to olefin polymerization procatalysts and thence to olefin polymerization catalysts by technology which is now largely conventional. The catalysts are high activity catalysts and are used to produce polyolefin having good properties. It would be of advantage, however, to produce improved procatalyst precursors and olefin polymerization catalysts whose use results in the production of polyolefin polymer of improved properties.